This is not true. PDF is a device-independent document format. Except
for bitmap images, there is no DPI measure at all, and it's wholly
your responsibility to use a large enough image to get 300 DPI.
Austin, its true at the end of the day you have X pixels by Y pixels
independent of DPI. But for some applications, they need the documents
size expressed in "inches".
In other words, when you go to document properties, it will currently
tell you the size based on 72 DPI, which will screw some applications up
if its setup to only look at the document size in inches.